The present invention relates to a method for removing DC offset from a digital baseband signal value pair included in a set of digital baseband signal value pairs and to an offset removing apparatus for performing such a method.
Such a method and offset removing apparatus are for instance described in the published UK Patent Application, publication NR GB 2 267 629. Therein a method of processing of a set of digital baseband signal-value pairs is described, whereby each pair of the set is made up of an inphase signal value and a corresponding quadrature signal value such that, if these are plotted on an I-Q diagram, the plotted pairs lie on a common circle, as is further described in this prior art document. The processing consists of averaging the inphase signal values of a set to produce an I-direction means value, and by performing the same operation on the quadrature signal value pairs to thereby producing a Q-direction means value. Further processing in the prior art document consists of subtracting the thus obtained respective average values from the individual inphase, respective quadrature, signal values, followed by other fine tuning steps, as extensively described in the prior art document, thereby aiming at removing the DC offset from the digital baseband signal. An arrangement including means for performing these processing steps, is as well described in this prior art document.
This method, as well as other signal averaging methods for determining the DC offset, which can be found in present literature, is however not always adequate, especially in case this DC offset can vary suddenly and substantially as a function of the time. A worst case situation being such that a first half of the signal-value pairs of such a set is subject to a relatively low DC offset value, whereas a second half is subject to a very high DC offset value or vice-versa. Indeed, in all these prior art DC offset cancellation systems, the DC offset is determined by performing an averaging operation using all samples of a burst incoming digital baseband signal value pairs. This burst thus corresponds to the set of digital baseband signal value pairs as is described in the preamble of the first claim of this document. In GSM systems however, due to high-blocking signals self-mixing, a sudden increase in the DC offset can occur in the middle of such a burst. Indeed, these high blocking signals originate from random access bursts generated by neighbouring terminals. If these are picked up by the antenna of a GSM burst mode receiver, these may leak to the local oscillator input of the mixer. Even though they are modulated at a carrier which frequency is laying far enough from the normal signal modulation carrier, they may become DC offset signals for the normal DC demodulated signals, due to leakage of the antenna of the demodulator, and subsequent so-called self-mixing within this demodulator circuit during the direct conversion demodulation of the carrier wave signal. This latter process is described in the published European patent application, publication number 0 594 894 A1. The amplitude of these dynamic blocking levels may exceed these of the normal signal bursts even with 80 dB, stated by the GSM specification 0505. Moreover, these can suddenly appear, the worst case situation being such that these signals appear in the middle of a normal signal burst, representing the aforementioned worst case situation, whereby the set of signal value pairs thus corresponds to a burst of signals received by the receiver.
Averaging methods which aim at determining the DC offset in the data signals, by averaging these signals over a complete burst, will thus result in a completely wrong determined DC offset for this worst case situation. Even very sophisticated averaging methods such as described in the last mentioned European patent application and in the published paper xe2x80x9cThe challenges for analogue circuit design in mobile radio VLSI chipsxe2x80x9d, written by Jan Sevenhans and Dirk Rabaey, published in Microwave Engineering Europe, May 1993, pp. 53-59, may therefore not be future safe enough, since the problem of the dynamic blocking level mixing DC offset may become worse with the increase in use of GSM mobile terminals. Indeed, with a widespread use of GSM terminals, the probability of occurrence of these self-mixing dynamic blocking levels increases considerably.
An object of the present invention is to provide a method and an offset removing apparatus of the above known type but allowing a dynamic and accurate determination of the dynamically changing DC offset, in a simple way.
According to the invention, this object is achieved due to the fact that said method further includes the steps as described in claim 1, and in that said offset removing apparatus is further adapted as described in claim 5.
In this way, by considering both the inphase and quadrature signal value pairs together as one point in the I-Q plane, and by two-dimensionally fitting this well-known geometrical figure through a subset of points in the I-Q plane, such that the center of this geometrical figure is obtained. Since this center corresponds to the DC offset, the DC offset can thus exactly be determined. In case this operation is repeated on subsequent ones of such sets or subsets within a complete burst, a jump in the DC offset can easily be discovered. By taking the subset such that the minimum amount of points, required for fitting the known figure is taken, the highest accuracy is obtained since this will reduce the amount of faulty detections. Since this solution does not require any feedback loop or a sophisticated algorithm, a very simple solution is thereby obtained.
Yet a further characteristic feature of the present invention is mentioned in claims 2 and 6.
When the thus determined DC offset coordinates are subsequently subtracted from the I, resp, Q signal values of the original data points, it is evident that also the DC offset will be more accurately removed.
Another characteristic feature of the present invention is described in claims 3 and 7.
In this way, a simple and accurate method for detecting and for removing DC offsets from at least one burst of received carrier wave modulated signals is obtained. The offset removing apparatus is thereby also adapted for removing DC offset of such bursts, as is described in claim 7.
Further characteristic features of the present invention are described in claims 4 and 8.
By selecting successive sets and subsets from a burst, and by repeating the aforementioned steps on subsequent ones of such sets, the accuracy of the determination of the DC offset can thus be significantly increased. A very simple variant could for instance consist of performing the fitting operation of a first set of samples selected in the beginning of the burst, and on a second set of samples selected from the end of the burst. By this simple method a jump in the DC offset can already be observed.
The present invention further also refers to a burst mode receiver incorporating such an offset cancellation means, as described in claim 9.